Wrap the meat tightly in plastic wrap and put it into the freezer for 45 minutes. It should be firm but not completely frozen.
Heat a large cast-iron skillet or a griddle over medium-high heat. Add 1 tablespoon of oil. Add the onions and peppers and season with salt and pepper. Cook until they are soft and lightly browned, 8 to 10 minutes. Reserve 1/2 cup cooked onions and peppers for another use, such as the Round 2 Recipe Chili Tostada.
In a medium pot over medium heat, add 1 tablespoon oil. Stir in the flour and cook for 1 to 2 minutes. Whisk in the milk and cook until it just starts to bubble and thicken, 4 to 5 minutes. Turn off the heat and whisk in the cheese and cayenne. Stir occasionally as the cheese melts. Cover and keep warm.
With a sharp knife, slice the meat into very thin strips. In a large skillet over medium-high heat, add the remaining tablespoon of oil and cook the meat until it is done to your liking, 1 to 3 minutes per side. Divide the onion and pepper mixture among the 4 rolls. Top with the meat and the cheese sauce.